1. Technical Field
The present invention relates to apparatus for and methods of locating and correcting unbalance in a body to be balanced. In its more particular aspects, the invention provides apparatus and method for measuring an unbalance in a body to be balanced by displaying on a screen representations of positions of the body as it rotates.
2. State of the Art
The process of balancing a rotor comprises two steps. The first step consists in measuring the unbalance in terms of its angular position and its magnitude during a test cycle. The second step consists in balancing out the unbalance determined during the test cycle by removing or adding material at particular points on the body to be balanced In the case of "polar balancing" the balancing must be effected in each balancing plane at the position which corresponds to the angular position of unbalance discovered during the test cycle. Consequently, it is necessary to rotate the body to be balanced into the corresponding working positions for correcting the unbalance.
For each working position, the part of the body at which material must be removed or added must be suitably placed for such a corrective operation, which is usually carried out by a metal working device. The rotating of the body to be balanced into the working position may take place on the same machine that was used for obtaining the measurements (balancing machine). However, the rotating of the body to be balanced into its working position may be performed on a special balancing machine to which the stored results of the measurements are conveyed. The "working" at the working position may consist in a direct balancing of the unbalance. The working may, however, consist in a marking of the spot where the unbalance occurs for a subsequent balancing operation.
The unbalance of a body to be balanced is usually measured dynamically at two axially displaced, radial planes, the "balance planes" with the aid of a balancing machine. The angular position and magnitude of the unbalance is determined for each of these planes. Force or displacement sensors deliver, via a known electric circuit or computer, a combined AC signal having the same frequency as the body to be balanced for each of these balance planes. The amplitude of the AC signal provides a measure of the magnitude of the unbalance. The phase of the AC signal provides a measure of the angular position of the unbalance. A known process of determining the angular position of an unbalance consists in comparing the AC signal with two reference AC signals which also have the same frequency as that of the body to be balanced and which are phase-locked therewith but which are 90.degree. out of phase with each other. Consequently, the AC signal can be multiplied with each of the reference signals at each time point.
Alternatively, each of the reference signals may be used to control a phase-sensitive demodulator and the AC signal can be mixed in the two phase-sensitive demodulators. If the body to be balanced is being driven by a motor which is phase locked with the rotation of the body, e.g. via a Cardan shaft, then the reference signals can be produced by a generator connected to this motor. However, if such a phase-locked type of driving connection is not available and the body to be balanced is being driven by a belt for example, or if the measurements are being taken when coasting, then a mark can be put on the body and this mark can be detected, (e.g. with a photoelectric detector). Such a mark and detector arrangement provides reference signals from which AC or rectangular pulse reference signals can be generated. In this way, one can obtain by suitable mixing of the AC signal and the reference signals (multiplication or pulsed rectification) two values, usually in the form of DC signals, which correspond to the unbalance components. The latter are expressed in terms of points of a co-ordinate system which is defined relative to the body and the angular position of the generator connected to the motor or the angular position of the mark on the body.
It is known to use the thus obtained components to produce a direct indication of the magnitude and angular position of the unbalance on a screen.
In one known indicating arrangement of this type, two watt meters are arranged at a mutual angular spacing of 90.degree. and receive the signals from the sensors sensing the centrifugal force exerted by the body to be balanced during its rotation. The watt meters are provided with mirrors which are also mutually angularly offset by 90.degree.. A light beam is directed to the two mirrors during rotation of the body to be balanced. The reflected light beam is directed to a diffused plate, e.g. ground glass screen, having a polar coordinate system inscribed thereon. The light beam is reflected by the two mutually perpendicular mirrors in accordance with the two mutually perpendicular components of the unbalance and generates a resultant point of light at an angular position in the polar magnitude of the unbalance. It is also known in a balancing machine of this type to store the light point indications so that they are maintained even after the ending of the test cycle and the stopping of the apparatus. It is also known to indicate the magnitude and the angular position of the unbalance by means of a cathode ray tube wherein the two signals corresponding to the unbalance components are used to control the electron beam in two mutually perpendicular directions.
In order to rotate the body to be balanced into its working position, a second light mark (position marker) is then created on the same screen. This second mark indicates the rotational positions assumed by the body during the rotating process. When the body has been so rotated that the second mark coincides with the stored mark, then the body is in its working position. This operation is carried out by rotating a projector or mirror conjointly with the body to be balanced. This projector or mirror is coupled in phase-locked relationship to the drive motor rotatingly driving the body to be balanced, by means of an "electric shaft" i.e. a rotating field system transmitter and receiver. The transmitter has a rotor which is coupled to the drive motor and the receiver has a rotor which is coupled to the projector or mirror. This system requires that the drive motor is coupled in phase-locked relationship to the body to be balanced.
If no such phase-locked driving connection is available, other systems are known for monitoring the rotation of the body to be balanced into its working position for correcting the unbalance. Such systems which are known from European Patents No. 0,074,416 and 0,085,873, do not rely upon indicating arrangements of the type as described hereinbefore.
From European Patent No. 0,074,416, a balancing machine is known in which a body to be balanced is driven by a belt, i.e. the drive motor does not rotate synchronously with the body. A pulse generator is coupled to the motor. A mark on the body to be balanced is detected by a pick-up device and delivers a reference signal at each revolution of the body. A counter counts the number of pulses generated by the pulse generator between each occurrence of a reference signal. In this way, the number of pulses corresponding to a full revolution of the body is determined. By multiplying this number by the ratio of the measured rotational angle of unbalance and 360.degree., the number of pulses corresponding to the angle of unbalance is obtained. Thereafter, the body to be balanced is rotated into its working position in which the reference mark is located at the working position. The body to be balanced is, then, further rotated until the counted pulse number is equal to a stored pulse number representative of the angular position of the unbalance relative to the reference mark or until such stored pulse number is counted down to zero.
From European Patent No. 0,085,873, a balancing machine is also known in which a pulse generator is coupled to an idler roller of the balancing machine. A mark is made on the body to be balanced. This mark is detected by a pick-up device and thereby produces a reference pulse. A measurement pulse is derived from the AC signal. The phase of the measurement pulse relative to the reference pulse provides an indication of the angular location of the unbalance relative to the mark. During a test cycle, the pulses emanating from the pulse generator are counted from the occurrence of the reference pulse up to the appearance of the measurement pulse. The resultant count is stored. The rotating of the body to be balanced is then accomplished with the aid of the pulse generator in accordance with the count.
The balancing machines according to European Patents No. 0,074,416 and 0,085,873 rely upon pulse counting and count comparison operations for rotating the body to be balanced into the working position. Due to the lack of a phase-locked driving connection between the rotary drive means and the body to be balanced, the aforenoted screen displaying functions can not be employed in such balancing machines for indicating the rotation of the body to be balanced into the working position for correcting the balance.
Also, there exist a number of cases in which a phase-locked driving connection between the body to be balanced and the rotary drive means would be undesirable for measuring and correcting the unbalance because the Cardan shaft which provides the phase-locked driving connection, would affect the result of the measurements due to the occurrence of laterally directed forces. In such cases it would be highly desirable to carry out such measuring and correcting operations without making use of the phase-locked driving connection even if it is present.